Various methods are known as a method for the preparation of fine particles of an oxide of a metal such as titanium, zirconium and the like.
Titanium dioxide, for example, is widely used in the fields of cosmetics, paints and other by virtue of the excellent weatherability and strong hiding power. Known methods for the preparation of titanium dioxide include the sulfuric acid process in which an aqueous solution of titanium sulfate is neutralized followed by calcination of the thus formed precipitates and the chlorine process in which titanium tetrachloride is pyrolyzed and oxidized at a high temperature. In these conventional methods for the preparation of titanium dioxide of rutile type, however, growth of particles takes place in the course of the manufacturing process so that the diameter of the thus produced titanium dioxide particles is so large as to exceed 1 .mu.m.
According to the disclosure by Funaki, Saeki, et al. in Kogyo Kagaku Zasshi, vol. 59, No. 11, page 1291 (1956), it was established that fine particles of anatase-type titanium dioxide can be produced by mixing titanium tetrachloride and water in the vapor phase at a temperature in the range from 200.degree. to 800.degree. C. or fine particles of anatase-type titanium dioxide containing or not containing a very small amount of rutile-type particles can be produced by the reaction of titanium tetrachloride and water in the liquid phase. These methods, however, can produce only particles of irregular shapes and no spherical particles can be obtained thereby.
As a method for preparing spherical particles of a metal oxide, on the other hand, there are known a method in which a hydrolyzable titanium (IV) compound in the form of a liquid aerosol is hydrolyzed by being contacted with water vapor in a dynamic flow (see U.S. Pat. No. 4,241,042 and a method in which a precursor of a metal oxide in the form of a very fine droplet suspension of the liquid is heated and gasified by evaporation and thermal decomposition and then contacted and reacted with an oxygen-containing gas in the vapor phase to give spherical fine particles of the metal oxide (see Japanese Patent Kokai 59-107904 and 59-107905).
The spherical metal oxide particles obtained in each of these methods, however, have an average particle diameter of at least 50 nm with, in addition, variations in the particle diameter.
In order to improved on the prior art, the inventors have previously proposed a method according to which titanium dioxide in a spherical and extremely fine particulate form can be prepared by the thermal decomposition of a titanium alkoxide (Japanese Patent Application laid-open 60-186418.)
Although spherical and ultra-fine particles of titanium dioxide can be formed in a relatively simple procedure according to this method, a problem therein is that the titanium dioxide particles finally obtained cannot be spherical and ultra-fine unless the concentration is extremely low due to the bonding and coalescence of the ultra-fine particles of titanium dioxide as formed.
The present invention relates to a method for the preparation of ultra-fine spherical particles of a metal oxide freed from the above described problems and the object thereof is to prepare ultra-fine spherical particles of a metal oxide, in the decomposition of a vaporizable metal compound to form ultra-fine particles of the metal oxide, by preventing coalescence of the particles.